Allow ValueVelocityTracker methods to be called out of order.

Summary:
This better guards against unexpected touch event streams.
Fixes #5

Reviewers: O2 Material Motion, O6 Material Android platform reviewers, #material_motion, randcode-generator

Reviewed By: randcode-generator

Tags: #material_motion

Differential Revision: http://codereview.cc/D3105

library/src/main/java/com/google/android/material/motion/gestures/ValueVelocityTracker.java

@@ -119,6 +119,10 @@ public void onGestureMove(MotionEvent event, float value) {
    * Must be balanced with a previous call to {@link #onGestureStart(MotionEvent, float)}.
    */
   public void onGestureEnd(MotionEvent event, float value) {
+    if (velocityTracker == null) {
+      return;
+    }
+
     addValueMovement(event, value);
 
     velocityTracker.computeCurrentVelocity(PIXELS_PER_SECOND, maximumFlingVelocity);
@@ -129,6 +133,10 @@ public void onGestureEnd(MotionEvent event, float value) {
   }
 
   private void addValueMovement(MotionEvent event, float value) {
+    if (velocityTracker == null) {
+      return;
+    }
+
     int valueMovementAction;
 
     int action = MotionEventCompat.getActionMasked(event);